1. Field of the Invention
The present invention relates to a method and an apparatus for treating a semiconductor wafer and, more particularly, to a semiconductor wafer cleaning technology that involves an ozone water cleaning process and an ultra-pure water rinsing process.
2. Description of Related Art
In recent years, much attention has been attracted to ozone water made by including ozone gas in ultra-pure water that is used as cleaning water in processes of manufacturing semiconductor wafer and semiconductor chips. Ultra-pure water refers to water having such a level of purity as the content of materials dissolved therein such as sodium, iron, copper and zinc is in a range of one billionth grams per litter (μg/liter) to one trillionth grams (ng/liter). Ozone water produces oxygen trough spontaneous decomposition in a strong oxidation reaction. As a result, use of ozone water to clean the semiconductor wafer makes it easier to process the waste water. Ozone water is made by mixing ozone gas into ultra-pure water.
As the sizes of devices formed on semiconductor chips become increasingly smaller, the semiconductor wafer is required to have higher degree of cleanliness.
Thus the ozone water is made from ultra-pure water of ever higher degree of purity. Such an ultra-pure water includes extremely reduced concentrations of total organic carbon content (TOC) as well as particulate matter and metallic impurities.
The total organic carbon content can be decreased by irradiating the ultra-pure water with ultraviolet ray.
Concentration of ozone gas in ultra-pure water decreases over time through diffusion into the atmosphere and spontaneous decomposition, in addition to being consumed through reaction of the ozone. Rate of decrease in ozone concentration with time may be represented by half life of ozone, that is the period of time over which the concentration of ozone dissolved in ultra-pure water decreases to one half of the original level. Half life of ozone in ultra-pure water is typically from 10 to 30 minutes, while it depends on the liquid surface area (area of gas-liquid interface). In case the ultra-pure water is contained in a container having a large opening such as cleaning bath, half life of ozone becomes about 2 to 5 minutes.
The applicant of the present patent application previously proposed a method of elongating the half life of ozone by adding a trace of organic solvent to ultra-pure water thereby to suppress the spontaneous decomposition of ozone, as described in Japanese Unexamined Patent Application, First Publication No. 2004-079649.
The method described in Japanese Unexamined Patent Application, First Publication No. 2004-4079649 makes it possible to manufacture ozone water having a longer half life of ozone.
The ultra-pure water used to make this ozone water includes particulate matter and metallic impurities in addition to the carbon compounds. Contents of these impurities are higher than in ultra-pure water that does not include ozone gas.
Methods of refining ultra-pure water known in the art include ultraviolet ray sterilization method and ultraviolet ray oxidization method. The ultraviolet ray sterilization method exterminates harmful bacteria included in the raw water by irradiating the water with ultraviolet ray. The ultraviolet ray oxidization method decomposes nitrogen and phosphorus included in the raw water by means of the energy of ultraviolet ray by irradiating the water with ultraviolet ray.
When ultra-pure water treated by the ultraviolet ray sterilization method is used in the chemical solution cleaning process or the ultra-pure water rinsing process to treat the semiconductor wafer, it was found that the degree of cleanliness of the semiconductor wafer is lower than that achieved by using ultra-pure water treated by the ultraviolet ray oxidization method.
The ultra-pure water refined by the ultraviolet ray oxidization method has higher degree of cleanliness than that of the ultra-pure water treated by the ultraviolet ray sterilization method.
Therefore, a higher degree of cleanliness of the semiconductor wafer can be achieved by using the ultra-pure water treated by the ultraviolet my oxidization method in the chemical solution cleaning process and ultra-pure water rinsing process for the treatment of the semiconductor wafer.
However, it is difficult to use the ultra-pure water treated by the ultraviolet ray oxidization method in all cleaning and rinsing processes including the ozone water treatment process, due to the problem of half life of ozone.
It is necessary to make ozone water that includes high concentration of ozone, in order to overcome the problem of half life of ozone in the ozone water cleaning process that uses ozone water made by the ultraviolet my oxidization method.
However, an exclusive manufacturing facility is required to manufacture the ozone water that includes high concentration of ozone, leading to another problem of increasing manufacturing cost.
The inventor of the present application completed the present invention upon the following finding. That is, a semiconductor wafer having a high degree of cleanliness can be obtained at a low cost by using ultra-pure water that has been treated by the ultraviolet ray sterilization method and has a somewhat high TOC value in the ozone water treatment process, using ultra-pure water that has been treated by the ultraviolet ray oxidization method and has a low TOC value in the chemical solution cleaning process where the ultra-pure water of high purity is used as the water to dilute a chemical solution and in the ultra-pure water rinsing process that requires a high degree of cleanliness.